A charge coupled device is well known as a converter operative to convert an optical image into a series of electric image signals. The charge coupled device is superior in size, weight, power consumption and reliability to the image camera tube and is capable of producing the electric image signals which are less liable to be influenced by an image distortion and by an image printing phenomenon. For these reasons, the charge coupled device is superseding the image camera tube in the field of the video tape recorder, camcorder as well as of the industrial camera. Moreover, since the charge coupled device is improved in the image resolution capability depending upon the development in the semiconductor manufacturing techniques, the charge coupled device is now used as an image sensor incorporated in the high resolution broadcasting camera.
The development in the semiconductor manufacturing techniques is conducive to a miniaturization the device size and to a high density integration, which results in a high resolution image sensor. However, the smaller the element in size, the less the amount of electric charges for each image signal. When the electric charges for the image signal are reduced, it is necessary to decrease the amount of electric charges due to noise for maintaining the sensitivity and the dynamic range. For this reason, research and development efforts are made for a noise elimination circuit capable of improving the signal-to-noise ratio for a high sensitivity image processor.
These efforts have resulted in various noise elimination techniques one of which is well known as a delay and differential noise elimination technique proposed by Nishida et al of NHK Science and Technical Research Laboratories. FIG. 1 shows the image processor provided with the noise elimination circuit using the delay and differential noise elimination technique. The image processor largely comprises a charge coupled device 1 and a noise elimination circuit 2. The charge coupled device 1 has a photoelectric transducing section 3, a driving circuit 4 coupled to the transfer port of the photo electric transducing section 3, and a buffer circuit coupled to the output node of the photo electric transducing section 3, and, on the other hand, the noise elimination circuit 2 has a differential amplifier circuit 6, a level adjusting resistor 7 with a relatively small in resistivity coupled between the inverted node of the differential amplifier 6 and the buffer circuit 5, a delay line 8 with a relatively large resistivity coupled between the non-inverted node of the differential amplifier 6 and the buffer circuit 5, a feedback resistor 9 coupled between the inverted node and the output node of the differential amplifier 6, a gate circuit 10 coupled at the input node thereof to the output node of the differential amplifier 6, and a pulse generating circuit 11 coupled to the control node of the gate circuit 10. The charge coupled device thus arranged produces a series of the electric image signals when an optical image falls upon the photo electric transducing section 3. With driving pulse signals supplied from the driving circuit 4, the photo electric transducing section 3 sequentially transfers the electric image signals from the photo electric transducing 3 to the buffer circuit 5 for a temporal storage, and the electric image signals are supplied in serial from the buffer circuit 5 to the noise elimination circuit 2 for improvement in the noise-to-signal ratio.
In detail, assuming now that a time frame T for each electric image signal (FIG. 2(a)) is divided into a resetting section RS, a feed through section FT, and a data section DT, the delay line 8 retards each electric image signal by a time interval TI to produce the delayed electric image signal as shown in FIG. 2(b). The intensities of small parts of the optical image are indicated by the differences D1, D2 and D3 in voltage level between the feed through sections FT and the data sections DT, respectively. When the electric image signals and the delayed electric image signals are supplied to the inverted node and the non-inverted node of the differential amplifier 6, the differential amplifier circuit 6 produces noise eliminated electric image signals each of which is modulated in amplitude on the basis of a difference due to the time interval TI. Then, the differences D1, D2 and D3 are reformed into the differences D1', D2' and D3' as seen from FIG. 2(c). The noise eliminated electric image signals are sampled with a transfer signal (FIG. 2(d)) fed from the pulse generating circuit 11 for extractions of the effective voltage levels, and the effective voltage levels are by way of example converted into time sequential image signals, respectively. By virtue of the time interval TI, each of the differences D1', D2' or D3' is produced as a difference voltage level between the feed through section FT of the delayed electric image signal and the data section DT of the original electric image signal, so that the differences D1', D2' and D3' are indicative of the actual intensities of the small parts of the optical image, respectively, even if noises ride on the electric image signals. Moreover, the effective voltage levels are extracted from the noise eliminated electric image signals by the gate circuit 10, the time sequential image signals are free from the folded noise components of the higher band inherent in the sample-and-hold circuit.
However, a problem is encountered in the prior-art noise elimination circuit in deterioration of the gain at a high frequency operation which is causative of a barrier for a higher resolution. This is because of the fact that a parasitic capacitance C is coupled to the inverted node of the differential amplifier 6. The parasitic capacitance is causative of a substantial amount of a time constant together with the feedback resistor 9 and, for this reason, allows the differential amplifier 6 to serve as an integrating circuit. The frequency response characteristics of the differential amplifier circuit 6 are shown in FIG. 3, and the cut-off frequency fc is calculated as follows EQU fc=1/2.times..pi..times.C.times.Rf
where Rf is the resistance of the feedback resistor 9. If one of the parasitic capacitance C and the resistance Rf is increased in value, the responsible bandwidth is caused to be narrow with respect to the electric image signal supplied to the inverted input node, and, accordingly, the electric image signal is less liable to be balanced with the delayed electric image signal. For this reason, the noise elimination capability is deteriorated at a higher frequency over the cut-off frequency fc.